1. Field of the Invention
The present invention relates to a rotary actuator and, more particularly to a rotary actuator in which a magnetic screw is formed on each piston and rod so that the rod will be rotated with the stroke of the piston.
2. Description of Related Art
There have been used various types of rotary actuators utilizing a fluid such as the compressed air as a source of driving force.
Conventional vane type rotary actuators and rack-and-pinion type rotary actuators, however, have such a problem that their bodies are required to be extended in a direction meeting at right angles with a rod, and accordingly the use of a larger rotary actuator is needed.
Furthermore, the vane type rotary actuator has such a problem that fluid leakage occurs between a seal and a vane.
The present applicant has proposed a rotary actuator using a screw for the purpose of solving the above-described problems.
FIG. 19 shows a ball-screw type rotary actuator as a prior art example, which has been proposed by the present applicant in Japanese Patent Laid-Open No. Sho 60-44607.
A rod 122 is rotatably supported on a couple of rotating bearings 128 through a cylinder body 129. At the central portion of the rod 122 is formed a stepped section 123, and around the stepped section 123 is formed a ball groove 124 in a spiral form. Inside the cylinder body 129 is slidably fitted a piston 121 moveable in an axial direction relative to the rod 122. To the piston 121 is fixedly installed a ball retaining member 126 for retaining a plurality of balls 125 as one unit. The balls 125 are inserted in the ball groove 124 of the stepped section 123. Also in a guide hole formed in the piston 121 is fitted a guide rod 127 provided on the cylinder body 129.
Therefore, as the piston 121 slides in relation to the rod 122, the rod 122 rotates through the balls 125.
The interior of the cylinder body 129 is divided into right and left chambers by the piston 121.
Next, the operation of the ball-screw type rotary actuator having the above-described constitution will be explained. When the compressed air flows into the right chamber of the piston 121, the piston 121 slides to the position shown in FIG. 19. At this time, the rod 122 is rotated clockwise, as viewed from the left, by the operation of the balls 125. Also, as the compressed air flows into the left chamber of the piston 121, the piston 121 slides as far as the position where the piston 121 contacts the right-hand wall of the cylinder body 129. At this time, the rod 122 rotates counterclockwise as viewed from the left through the balls 125.
Next, the construction of the screw type rotary actuator is shown as the second prior art example in FIGS. 20 and 21, which has been proposed by the present applicant in Japanese Utility Model Laid-Open No. Sho 62-162401.
A rod 132 mounted through a cylinder body 139 is rotatably supported on two rotating bearings 138. At the central part of the rod 132 is formed a step-like external screw section 133.
In the cylinder body 139, a piston 131 is so fitted as to be axially slidable on the rod 132. An internal screw member 136 engaged with an external screw section 133 is fixedly installed as one unit to the piston 131. The piston 131, as shown in FIG. 21, is elliptical in a sectional form, so that it will not rotate.
Therefore, with the sliding of the piston 131 on the rod 132, the rod 132 is rotated by the operation of the external screw section 133 and the internal screw member 136.
The interior of the cylinder body 139 is separated into the right and left chambers by the piston 131.
Next, the operation of the screw type rotary actuator having the above-described constitution will be explained. With the inflow of the compressed air into the left chamber of the piston 131, the piston 131 slides as far as a position shown in FIG. 20. At this time, the rod 132 is rotated counterclockwise as viewed from the left, by the operation of the external screw section 133 and the internal screw member 136. Also with the inflow of the compressed air into the right chamber of the piston 131, the piston 131 slides as far as a position in which the piston 131 contacts the left wall of the cylinder body 139. At this time, the rod 132 is rotated clockwise as viewed from the left side, by the operation of the external screw section 133 and the internal screw member 136.
However, the prior art ball-screw type rotary actuator and the screw type rotary actuator have the following problems.
That is, because of a low torque transmission efficiency of the screw, it is demanded to build a large-sized rotary actuator capable of producing a required torque. Besides, since the ball screw transmits the torque through a local contact surface, an increased surface pressure and accordingly a short life will result.
To solve these problems of the prior art ball-screw type rotary actuator and the screw type rotary actuator, the present applicant has suggested the adoption of a magnetic screw in place of the screw and the ball screw types in Japanese Patent Laid-Open No. Sho 62-46005.
In FIG. 22 is shown the constitution thereof. A rod 141 mounted through a cylinder 146 is provided with a stepped section at the center, and a spiral magnetized band 142 is formed on the surface. In the meantime, in the groove 144 of a piston 143, two guide magnets 145 which are permanent magnets are embedded and fixedly attached. The two guide magnets 145 slide within the cylinder 146 together with the piston 143.
Furthermore, the laid-open gazette tells, on the 12th through 16th lines in the left lower column on page 3, "as a modification of the present embodiment, a spiral guide magnet may be provided on the inner periphery of the center hole of the piston, and a block type driven magnet may be provided on the outer periphery of the rotor. In this case, also the rotor rotates with the up-and-down stroke of the piston."
In the above-described constitution, the rod 141 rotates with the sliding of the piston 143.
According to the laid-open gazette (3rd through 11th lines at the right upper column on page 4), this invention has such an advantage that the rotary actuator of the present invention needs no high sealing technique because those parts which make relative movement at the time of driving are properly sealed with an O-ring; and since a spiral permanent magnet is used on one side of permanent magnets attracting each other to rotate the output shaft, the maximum rotation angle of the output shaft can be increased to 360 degrees or over by providing a small lead of spiral groove or by extending the cylinder in an axial direction.
The prior art magnetic screw type rotary actuator disclosed in Japanese Patent Laid-Open No. Sho 62-46005, however, has the following problems.
(1) Since a spiral permanent magnet is provided on the rod, or on one side of the permanent magnets in the piston hole, and a square block type permanent magnet is provided on the other side, the rod rotates unsmoothly and nonuniformly during the sliding of the piston.
(2) Since the permanent magnet is only partly fitted in the piston, there exist both the angle of attraction and the angle of non-attraction of the permanent magnets, resulting in nonuniform rotation of the rod.
Furthermore, the transmission torque at the magnetic screw, that is, a coupling power between the rod and the piston, is small; the use of a large type apparatus, therefore, becomes necessary to obtain the coupling power required; otherwise the above-described problem of the prior art rotary actuator cannot be solved.
(3) A spiral magnetized band formed on the rod is composed of laminated permanent magnets, in which no accurate magnetized band can be formed because of an inaccurate continued section, causing unsmooth, nonuniform rod rotation.
Furthermore, there are such problems as an increased number of assembling processes and a high cost.
Particularly when the permanent magnets are bonded to the inner periphery of the piston, joints tend to contact the rod, resulting in unsmooth rod rotation.
(4) The O-ring 140 is used as a sealing material for the purpose of preventing air leakage between the piston 131 and the rotating rod 132; however, in the case of the magnetic screw type rotary actuator, which employs a magnetic screw type coupling, has not so great a coupling power. Therefore, if an elastic member such as the O-ring 140 is held in direct contact for sealing with the rotating rod 132, the effective torque of the rotating rod 132 will decrease due to the effect of frictional force of the O-ring 140.
(5) Furthermore, when an elastic seal like the O-ring 140 is used, the frictional force varies with a change in the pressure of the compressed air which is a working fluid, or with a change in the rotational speed of the rotating rod 132, resulting in a variation in the output torque. This is disadvantageous in keeping a constant output torque.
(6) For detecting the angle of rod rotation, the rotational position of a hand mounted on the rod is directly detected by means of a limit switch. It is, therefore, necessary to mount a limit switch or the like in the rotational position of the hand; the limit switch, however, is very inconvenient because it is likely to interfere with hand manipulation and to decrease a surrounding space.
Furthermore, to detect the rotational position of the rod from time to time, it is necessary to mount a rotation angle detector such as a rotary encoder in connection with the rod. The rotary encoder, however, is large in size, inconvenient for mounting, and besides expensive.